Dehydrocyclodimerization is a process in which aliphatic hydrocarbons containing from 2 to 6 carbon atoms per molecule are reacted over a catalyst to produce a high yield of aromatics and hydrogen, with a light ends byproduct and a C2–C4 recycle product. This process is well known and is described in detail in U.S. Pat. Nos. 4,654,455 and 4,746,763 which are incorporated by reference. Typically, the dehydrocyclodimerization reaction is carried out at temperatures in excess of 500° C. (932° F.), using dual functional catalysts containing acidic and dehydrogenation components. The acidic function is usually provided by a zeolite which promotes the oligomerization and aromatization reactions, while a non-noble metal component promotes the dehydrogenation function. One specific example of a catalyst disclosed in U.S. Pat. No. 4,746,763 consists of a ZSM-5 type zeolite, gallium and a phosphorus containing alumina as a binder.
The conditions used for the dehydrocyclodimerization reaction result in catalyst deactivation which is believed to be caused by excessive carbon formation (coking) on the catalyst surface. After several days (from about 3 to 10 depending on the operating temperature) enough activity has been lost due to coke deposition that regeneration of the catalyst is necessary. Regeneration involves burning or oxidizing the coke present on the catalyst at elevated temperatures. In addition to loss of activity due to coke formation, catalysts containing a phosphorus modified alumina as a binder are gradually deactivated (over a period of time from several months to about a year) by exposure to hydrogen at temperatures generally greater than 500° C. (932° F.) and particularly greater than 565° C. (1049° F.). This loss of activity due to hydrogen exposure cannot be restored by regeneration means, i.e., burning or oxidation at elevated temperatures. Therefore, a process is sought for reactivating catalysts that have lost activity due to exposure to hydrogen above 500° C. (932° F.).
As used in this application, regeneration refers to the process of restoring lost activity due to coke formation, while reactivation refers to the process of restoring lost activity due to hydrogen exposure.
U.S. Pat. No. 5,212,127 (Kocal et al.) describes a process for restoring substantially all of the activity of a dehydrocyclodimerization catalyst that has been deactivated by exposure to hydrogen at high temperature. This patent teaches treating the catalyst with an aqueous solution of a weakly acidic ammonium salt or a dilute acid solution, rinsing the treated catalyst, and oxidizing the rinsed catalyst to produce reactivated catalyst. The treating solution should contain from about 0.1 to about 5 moles/liter of the salt or acid, and the preferred trailing solution is an ammonium nitrate solution.
One of the problems with the reactivation process in U.S. Pat. No. 5,212,127 is the use of the salt or the acid in the treating solution. Expenses arise as a result of purchasing, handling and ultimately disposing of the salt or the acid. In addition, treating with the salt or the acid necessitates a rinsing step after treating and prior to oxidizing. Therefore, a process is sought for reactivating catalysts that have lost activity due to exposure to hydrogen above 500° C. (932° F.), wherein the process does not require a salt or acid in the treating solution. Preferably, the process should be easily incorporated and employed in commercial catalytic dehydrocyclodimerization processes.